Field of the Invention
The present invention relates to a control technique for a voice coil motor.
Description of the Related Art
Digital still cameras, digital video cameras, or electronic devices (e.g., cellular phones) having an image acquisition function include an actuator for positioning a focusing lens. Known examples of such actuators include actuators using a stepping motor method, actuators using a piezoelectric method, and actuators using a voice coil motor (VCM) method.
A VCM is capable of generating driving power in a linear direction that corresponds to the direction of a current that flows through its coil. Known examples of such a VCM driving method include a spring return method and a bi-directional driving method.
With a spring return mechanism VCM, a driving current is supplied to a coil so as to generate a driving force in a first direction. Furthermore, a driving force is generated in a second direction that is the opposite of the first direction by means of a spring coupled with a moving element. That is to say, the spring return mechanism VCM is configured as a combination of an electrical driving mechanism and a dynamic driving mechanism. In a case of employing such a spring return mechanism VCM as a driving source, such an arrangement requires only a driving current that flows through the coil in a single direction, thereby allowing the driving circuit to have a simple configuration.
In contrast, in the bi-directional driving method, such an arrangement employs a driving circuit such as an H-bridge circuit that is capable of switching the driving current between a source current and a sink current on respective ends of the VCM. With the bi-directional driving method, such an arrangement is capable of switching the direction of the coil current, thereby allowing the driving force to be provided in both the positive direction and the negative direction.
With the spring return method, the VCM requires no driving current supply when the position of the VCM is zero (reference position). In contrast, with the bi-directional driving method, the driving current for the VCM can be set to substantially zero even if the reference position is set to a desired value. Thus, such an arrangement has an advantage from the viewpoint of low power consumption.
A driving circuit according to a conventional bi-directional driving method is configured to have left-right symmetry. With the direction of a current that flows through the coil of the VCM in the first direction as a positive direction, the driving circuit is configured to generate a driving current in a range between −IMAX and IMAX with a current value of zero as the center.
However, in some cases, there is a difference in the optimum driving current range between manufacturers of VCMs to be driven, between models of VCMs, or otherwise between sets mounting such VCMs.